Bonded magnets molded by binding magnet powders of a rare earth alloy by means of a resin binder contain the resin binder and thus have poorer magnetic properties than those of binderless sintered magnets, but are easy to process into any shape and also excellent in dimensional accuracy, and therefore are used in various applications. For example, as regards their use in sensors detecting the angle in a non-contact manner, bonded magnets are utilized as sensor magnets detecting the opening/closing angle in flow channel switching valves for water pumps for effectively cooling engines, inverters, batteries, etc. of HEVs and EVs, oil pumps, fuel pumps and the like in the automobile field, and are utilized as sensor magnets for detection of the absolute angle in robots in the industrial machinery field.
Bonded magnets are classified into those obtained by charging a mixture comprising a magnet powder and a resin binder such as a thermosetting epoxy resin into a mold for compression molding thereof (compression-bonded magnets) and those obtained by pelletizing a mixture of a magnet powder and a thermoplastic resin binder and injection-molding this pelletized mixture. Compression-bonded magnets can contain a large amount of magnet powder as compared with injection-molded magnets, and thus can attain high magnetic properties.
When rare earth magnet powders are used in bonded magnets, the magnets contain iron or a rare earth element, and thus involve the problem of internal penetration of rust, or are likely to be deteriorated in magnetic properties by oxidation corrosion. Especially, such problems become pronounced in a corrosive environment in which the bonded magnets are in contact with a fluid such as water. Therefore, in the bonded magnet, a resin coating is formed on an exposed surface of the magnet, for example, by electrodeposition coating, electrostatic coating or spray coating to cope with the above problems.
Conventionally, there has been proposed a compression-bonded magnet production method in which a rust-proof thermosetting coating is formed on the surface of a rare earth magnet by an immersion method (see Patent Document 1). This production method involves repeating a cycle of immersion, drying and curing twice to six times to form a rust-proof thermosetting coating with a size of 0.005 mm to 0.05 mm on the surface of a magnet while impregnating a resin into the voids formed in the magnet.